A 1-car garage needs a 9,000–12,000 BTU mini split, a 2-car garage needs 18,000–24,000 BTU, and a 3-car garage needs 24,000–36,000 BTU — but insulation level is the deciding factor that can shift these numbers by 40%. An insulated, sealed 2-car garage can get by with an 18,000 BTU unit, while an uninsulated 2-car garage with a leaky door in Phoenix might need 30,000 BTU or more.
Garages are uniquely challenging to condition: large overhead doors with minimal insulation, concrete slab floors that absorb heat, poor air sealing, and often no insulation in walls or ceiling. This guide accounts for all of these factors.
Garage Mini Split Sizing Chart
| Garage Type | Size (sq ft) | Insulated + Sealed | Insulated Only | Uninsulated |
|---|---|---|---|---|
| 1-car | 200–300 | 9,000 BTU | 12,000 BTU | 12,000–15,000 BTU |
| 1.5-car | 300–400 | 12,000 BTU | 12,000–15,000 BTU | 15,000–18,000 BTU |
| 2-car | 400–576 | 18,000 BTU | 18,000–24,000 BTU | 24,000–30,000 BTU |
| 2.5-car | 576–700 | 18,000–24,000 BTU | 24,000 BTU | 30,000 BTU |
| 3-car | 700–900 | 24,000 BTU | 24,000–30,000 BTU | 30,000–36,000 BTU |
| 3-car deep/tandem | 900–1,200 | 30,000–36,000 BTU | 36,000 BTU | 36,000–48,000 BTU (2 units) |
"Insulated + Sealed" means: R-13+ walls, R-30+ ceiling, insulated garage door (R-8+), weather stripping on all door edges, and sealed gaps/penetrations. This is the gold standard for a conditioned garage and reduces your required BTU by 30–40% compared to uninsulated. The garage door insulation alone makes the biggest single difference.
Why Garages Need More BTU Per Square Foot
Regular rooms need 18–25 BTU per square foot. Garages need 30–55 BTU per square foot because of several compounding factors:
| Factor | Impact on Cooling Load |
|---|---|
| Garage door (uninsulated) | +25–40% — a standard steel door is R-0 to R-2 |
| Garage door (insulated, R-8+) | +10–15% — much better but still the weakest wall |
| Concrete slab (full sun) | +10–15% — absorbs and radiates heat for hours |
| No wall insulation | +20–30% — exposed studs and single-layer siding |
| Poor air sealing | +15–25% — gaps around door, windows, and penetrations |
| Dark roof directly above | +10–20% — radiant heat from roof deck |
| Frequently opened door | +10–20% — every opening floods in outdoor air |
A fully uninsulated garage in Zone 2 (Phoenix, Houston) can require 50–60 BTU per square foot — nearly 3× the standard residential rate.
Climate Zone Adjustments for Garages
| Climate Zone | Cooling Multiplier | Heating Multiplier | Notes |
|---|---|---|---|
| Zone 2 (Hot) | 1.25–1.40 | 0.80 | Extreme summer heat in garages |
| Zone 3 (Warm) | 1.10–1.25 | 0.90–1.00 | Near-baseline |
| Zone 4 (Mixed) | 1.00 | 1.00–1.10 | Balanced needs |
| Zone 5 (Cool) | 0.90 | 1.15–1.30 | Heating dominates |
| Zone 6 (Cold) | 0.85 | 1.30–1.50 | Must use cold-climate mini split |
| Zone 7 (Very Cold) | 0.80 | 1.50–1.80 | May need supplemental heat |
Sizing Examples
Example 1: 2-car garage workshop in Austin, TX (Zone 2)
Dimensions: 20' × 24' = 480 sq ft. Walls insulated (R-13), ceiling insulated (R-19), insulated garage door (R-8). Weather-stripped but door opens 2–3 times daily. Workshop use: table saw, compressor, lighting (1,500W equipment heat).
Calculation: 480 × 35 BTU/sq ft (Zone 2, insulated garage) = 16,800. Equipment heat: +5,000 BTU. Frequent door opening: +15% = 25,070 BTU.
Recommendation: 24,000 BTU (2-ton) mini split. A Mitsubishi MSZ-GL24NA or equivalent handles this load with margin for Austin's 105°F peak days. Total installed cost: $2,800–$4,200.
Example 2: 1-car garage gym in Charlotte, NC (Zone 3)
Dimensions: 12' × 20' = 240 sq ft. Uninsulated walls and ceiling. Basic non-insulated garage door. One person exercising (1,200–1,800 BTU body heat during workouts).
Calculation: 240 × 45 BTU/sq ft (Zone 3, uninsulated) = 10,800. Body heat during exercise: +1,500 BTU = 12,300 BTU.
Recommendation: 12,000 BTU (1-ton) mini split. Perfect match. For year-round gym use, choose a heat pump mini split rated to at least 17°F for Charlotte's winter design temperature. A Fujitsu 12RLS3Y or Mitsubishi MSZ-GL12NA handles both heating and cooling.
Example 3: 3-car insulated garage in Denver, CO (Zone 5)
Dimensions: 32' × 24' = 768 sq ft. Fully insulated (R-13 walls, R-38 ceiling, R-12 garage door). Well-sealed. Used as car storage + occasional workshop. Main concern: heating in winter.
Cooling: 768 × 28 BTU/sq ft = 21,504 BTU. Heating: 768 × 50 BTU/sq ft (Zone 5 garage) = 38,400 BTU. Cold-climate HP capacity at 5°F: a 30K unit delivers ~22,000 BTU.
Recommendation: 36,000 BTU (3-ton) cold-climate mini split. The heating load drives the sizing. A 36K cold-climate unit delivers approximately 28,000 BTU at 5°F outdoor temp — covering 73% of the heating load. For the coldest days, supplement with a 1,500W portable heater or accept slightly lower garage temperatures.
Alternatively, two 18,000 BTU heads provide better air distribution in the 768 sq ft space.
Example 4: Detached uninsulated 2-car garage in Miami, FL (Zone 1)
Dimensions: 22' × 22' = 484 sq ft. Zero insulation. Metal roof. No weather stripping. Dark-colored exterior. Used as workspace 6+ hours daily.
Calculation: 484 × 55 BTU/sq ft (Zone 1, uninsulated, metal roof penalty) = 26,620 BTU.
Recommendation: 30,000 BTU mini split — OR — insulate first, then 18,000 BTU.
Insulating the garage (walls + ceiling + door) for $2,000–$3,500 drops the required BTU to approximately 16,000–18,000, allowing an 18K unit instead of 30K. The 18K unit costs $1,000+ less to purchase and $100–$200/year less to operate. In this case, insulating first saves money compared to upsizing the mini split.
Insulate First, Then Size: The Cost-Effective Approach
| Insulation Upgrade | Typical Cost | BTU Reduction | Payback Period |
|---|---|---|---|
| Garage door insulation kit (R-8) | $100–$300 | 20–30% | 1–2 years |
| Wall insulation (R-13 batts) | $500–$1,500 | 15–25% | 2–3 years |
| Ceiling insulation (R-30) | $400–$1,200 | 10–20% | 2–3 years |
| Weather stripping (all doors) | $50–$150 | 10–15% | <1 year |
| Total package | $1,050–$3,150 | 40–60% | 1.5–3 years |
Every dollar spent on insulation reduces the required mini split size (saving $200–$1,000 on equipment) and reduces annual operating costs ($50–$150/year). For uninsulated garages, insulating before installing a mini split almost always makes financial sense.
Garage Heating: Winter Sizing Considerations
If you're using your garage mini split for heating too (workshop, gym, living space conversion), the heating load often exceeds the cooling load in Zones 4–7.
| Garage Type | Zone 4 Heating BTU | Zone 5 Heating BTU | Zone 6 Heating BTU |
|---|---|---|---|
| 1-car insulated | 10,000–14,000 | 14,000–18,000 | 18,000–24,000 |
| 2-car insulated | 18,000–24,000 | 24,000–36,000 | 36,000–48,000 |
| 3-car insulated | 28,000–36,000 | 36,000–48,000 | 48,000+ (2 units) |
| 1-car uninsulated | 15,000–20,000 | 20,000–30,000 | Not practical |
| 2-car uninsulated | 28,000–40,000 | 40,000–60,000 | Not practical |
Heating an uninsulated garage in cold climates is extremely expensive. The heat loss through uninsulated walls, ceiling, and especially the garage door is enormous. A mini split running full-blast in an uninsulated 2-car garage in Zone 6 can cost $200–$400/month in electricity. Insulate the garage before attempting to heat it.
Mini Split Placement in Garages
For single-head installations, mount the indoor unit on the wall opposite the garage door, 7–8 feet high. This creates the longest air throw across the space, pushing conditioned air toward the door (the biggest heat gain source) and ensuring even distribution.
For 3-car garages or deep garages over 30 feet, consider two indoor heads: one on each side wall or one mid-depth on each side. A single head struggles to condition a space more than 25 feet from the unit.
The outdoor unit should be mounted on an exterior wall or a ground stand, avoiding the south/west side of the building (which gets the hottest sun exposure) when possible. Ensure at least 24 inches of clearance around the outdoor unit for airflow.
Key Takeaways
- Standard sizes: 1-car = 9K–12K BTU, 2-car = 18K–24K BTU, 3-car = 24K–36K BTU. Insulation level shifts these by ±40%
- Garages need 30–55 BTU/sq ft — 1.5–2.5× more than regular rooms — due to doors, slab, and poor insulation
- Insulating first (especially the garage door) reduces required mini split size by 30–40% and saves money long-term
- For year-round use in Zones 5+, choose cold-climate mini splits and size for the heating load
- In hot climates with uninsulated garages, spend the money on insulation rather than upsizing the mini split
- Two smaller heads beat one large head for garages over 700 sq ft or deeper than 30 feet
Frequently Asked Questions
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